1. Field of the Invention
This invention relates generally to methods and apparatus for carrying out ion implantation with decelerated ion beams. Specifically, this invention relates to an improved methods and new configuration of implanter by employing striking means for the measurement and control of implant angles and to reduce cross contaminations.
2. Background
The ion implantation processes is often limited by a technical difficulty that the implant angle of the decelerated ion beam is not accurately measured and controlled. Another difficulties is related to the problems caused by cross contaminations when the ion implanter is often used to implant different kinds of ions. These technical difficulties and limitations can be better understood and appreciated by further discussions of the technical background involved in the processes and configurations of ion implanters as currently employed to perform the ion implantation tasks.
Ion implantation is a ballastic process used to introduce atoms or molecules, called dopants, to make materials with useful properties. In particular, ion implantation is a common process used in making modern integrated circuits. The amount of ion beam current that can be transported in a conventional ion implanter depends on the ion beam energy and at low energies becomes unfeasibly low.
For a conventional high current ion implanter, an ion beam is extracted from an ion source and travels through a mass analyzer magnet to select specific ionic species. The selected or filtered ion beam emerges from the magnet and is then incident toward the semiconductor target wafers. The travel distance from the source to the wafers is usually about two meters. For an ion beam with an energy as low as 0.2 keV and beam currents as high as 10 mA, the space charge of the beam is so intense that the ion beam starts to blow up severely as it leaves the source. This problem exists regardless of what kind of beam focusing optics is used. After the ion beam travels about two meters there is not much usable beam current left for implantation. An efficient way to obtain high beam currents at low ion energy is to decelerate an ion beam from higher energy, e.g., 5 keV, to a lower energy, e.g., 1, 0.5, or as low as 0.2 keV, at a region close to the wafers. Although the beam may also blow up after deceleration, there is still sufficient beam current remaining for implantation because the distance between the deceleration region and the wafers is usually less than 0.4 meters. With the use of a plasma or electron shower, the beam blow-up will be less and beam transmission can be improved.
The above method is able to achieve high beam currents at energies below 5 keV by extracting ions at a higher than desired final energy, conducting a mass analysis of the ions, and then decelerating the ion beam to the desired energy just before it reaches the target. However, high-energy neutrals can be generated in the region between the mass analyzer and the deceleration electrodes when higher energy ions have charge exchange interactions with residual gases in the beamline. These neutralized atoms will not be decelerated by the decelerating electric fields and will reach the wafers at higher than desired energies. This results in what is known as energy contamination, which causes a deeper than desired dopant depth profile. Energy contamination is only tolerable to ˜0.1% in order to provide sufficient margin against shifts in device performance [L. Rubin, and W. Morris, “Effects of Beam Energy Purity on Junction Depths in Sub-micron Devices”, Proceedings of International Conference on Ion Implantation Technology, 1996, p96]. To have such a low neutral fraction it requires that the chamber pressures be kept very low (5.0E-7 torr) so as to minimize the probability of charge exchange reactions. This level of pressure is, however, very difficult to maintain under normal operating conditions in an implantation system due to the out-gassing of the photo-resist coating of patterned devices and the presence of feed gases from the source and plasma shower. Another issue is the variation in the level of contamination. Pressure fluctuations during the implant can cause across wafer effects. Day-to-day changes in residual vacuum or photo-resist quality can cause batch-to-batch effects. Finally, the potential loss of wafers worth millions of dollars exists due to these types of undetected vacuum problems.
In order to prevent severe consequences resulting from energy contamination Adibi et al have invented a device to monitoring high-energy neutral contamination in an ion implantation process (U.S. Pat. No. 5,883,391). Although, the device disclosed by Adibi et al. may be useful to monitor and prevent damages resulted from contamination of neutral particles, the device however does not provide a technical solution that can produce a positive effect of reducing the energy contamination.
For the purpose of reducing energy contamination in decelerated beam implant, England (U.S. Pat. No. 5,969,366) discloses a method of installing a magnet in between deceleration electrodes and the implant target. The major obstacle of implementing this approach to an ion implanter is distance between the deceleration electrodes and the implant target is increased. Consequently, the production worth low energy beam currents cannot be properly delivered to the target. The added distance between the deceleration electrode and the target therefore degrades the performance of the implanter disclosed by England.
For the above reasons, in order to project a low energy high current ion beam to the target wafer, it is often required to deflect and guide the beam along a curved trajectory as will be discussed in FIG. 1 below. In the processes of bending and guiding the trajectories of the ion beam, the ion beam incident angle as that projected onto the target wafer is changed. The angular shift, however, is an important parameter to measure and control for the uniformity of the ion implantation but the conventional configuration and implant processes still lack an effective method to accurately measure the ion beam incident angle. Furthermore, an implanter is often applied for implanting different kinds of ions. Meanwhile, an ion implanter has an Faraday that is usually placed behind the target wafer with an ion collection surface to function as an ion beam current monitor as well as an ion beam dump. The ion collection surface often absorbs and contains different kinds of implanted particles. However, different kinds of the implanted particles as residual particles contained in the Faraday may be sputtered away from the ion collection surface when bombarded by the incident ions. Some of the residual sputtered particles may land on the surface of the target wafer and cause cross contamination and thus adversely affect the purity and quality of the ion implantation operations.
Therefore, a need still exists in the art of ion implantation to design an improved configuration and methodology to accurately measure the incident angle of the ion beam guided through curved trajectories when incident onto the target wafer and also to prevent cross contaminations when the implanter is employed for implanting different kinds of ions onto many kinds of wafers.
Furthermore, since the traditional techniques of ion implantation using conventional deceleration approaches as described above does not provide a viable solution for very low energy ion implantation. There is a need in the art of IC device fabrication to provide new systems to provide very low energy implants with minimal energy contamination. In order to manufacture devices that require shallow p-type and n-type junctions, new methods and systems are required to resolve the difficulties and limitations of low energy ion implantation with effective control over energy contamination.